Perl Note: most of these links are from 2015 and 2016. Please submit new ones if you've got good suggestions.
srfi-1, the list library for Scheme
Hoogle, finding functions in standard Haskell libraries
core-api, Clojure core function docs
JavaScript:
Professor Frisby's Mostly Adequate Guide to Functional Programming (in javascript) (HN). Also see:
Python:
Functional programming in C++ (John Carmack) (alternative link)
Lua Functional (docs, HN) (also, functional-lua)
The FSet Functional Collections Libraries (Common Lisp) (Github)
Introduction to Functional Programming: the interesting part here may be that this course is said to show "that good OOP code is actually functional". (I haven't verified, but would expect it to show why you want your objects to be immutable, which applies to the FP::Struct model.)
Purely Functional Data Structures for the Impure (by osmaferon) (source)
Pure UI, on purely functional composition of a user interface (using the React JavaScript library)
Write code that is easy to delete, not easy to extend: even though the article doesn't mention functional programming, it helps this aim (pure functions don't depend on state, hence dependencies are on code only)
in which we plot an escape from the quagmire of equality, nice citation: (...) referential transparency allows you to have much greater confidence that your code is correct. Without it, the best you can say is "as long as the rest of this program behaves itself, this function should work". This works a lot like older OSes with cooperative multitasking and no process memory isolation; also discusses equality, and proposes adding immutable data types to Emacs.
The Problem With Single-threaded Shared Mutability, including the citation: My intuition is that code far away from my code might as well be in another thread, for all I can reason about what it will do to shared mutable state.
How I Write Tests (Functional Perl helps satisfy most of these points; "Write lots of fakes" may really be asking for a monad abstraction.) (HN), also Design for Testability
See how well Test::RandomCheck works. Look for inspirations from Hypothesis, a Python testing framework for property based testing.
The list monad in Perl and Python (by mjd, author of Higher-Order Perl) (HN)